Study helps us understand evolution of marine mammal communication

Until recently it was believed that the solitary nature of Araguaian dolphins meant that they wouldn’t have much use for communication. But scientists have now documented hundreds of sounds made by the dolphins — and they say that these vocalizations could help us better understand the evolution of underwater communication among marine mammals.

Using underwater cameras and microphones to record interactions between the dolphins, researchers recorded 20 hours of vocalizations, which they classified into 13 different types of “tonal sounds” and 66 types of “pulsed calls.” In total, they identified 237 distinct types of calls.

The most common sounds the dolphins made were “short two-component calls,” the researchers report in the study. About 35 percent of these calls were made by calves while reuniting with their mothers, which suggests that the calls are an important component of mother-calf communication.

The Araguaian river dolphin was just discovered and described to science in 2014, so it’s no surprise that there’s a lot we don’t know about the cetacean native to the Araguaia and Tocatins rivers in Brazil.

Until recently it was believed that the solitary nature of Araguaian dolphins meant that they wouldn’t have much use for communication. But scientists have now documented hundreds of sounds made by the dolphins — and they say that these vocalizations could help us better understand the evolution of underwater communication among marine mammals.

Though the waters of the Araguaia and Tocatins rivers are relatively clear, Araguaian dolphins, known locally as botos, are difficult to even locate, let alone study in the wild. The animals are quite skittish and usually flee when in the presence of humans.

But Gabriel Melo-Santos, a biologist from the University of St Andrews in Scotland, discovered that many of the dolphins frequent a fish market in the Brazilian town of Mocajuba, where shoppers typically feed the dolphins. Melo-Santos led a team of researchers that took advantage of the clear river water and the dolphins’ regular visits to get an up-close look at the animals’ interactions and behavior. The team detailed their findings in a study published in the journal PeerJ last month (the study was first published in PeerJ Preprints last year).

Using underwater cameras and microphones to record interactions between the dolphins visiting Mocajuba, Melo-Santos and team recorded 20 hours of vocalizations, which they classified into 13 different types of “tonal sounds” and 66 types of “pulsed calls.” In total, they identified 237 distinct types of calls.

The researchers say their findings show that Araguaian dolphins’ acoustic repertoire is anything but simple. “We found that they do interact socially and are making more sounds than previously thought,” Laura May Collado, a biologist at the University of Vermont and co-author of the paper, said in a statement. “Their vocal repertoire is very diverse.”

The most common sounds the dolphins made were “short two-component calls,” the researchers report in the study. About 35 percent of these calls were made by calves while reuniting with their mothers, which suggests that the calls are an important component of mother-calf communication. According to May Collado, “marine dolphins like the bottlenose use signature whistles for contact, and here we have a different sound used by river dolphins for the same purpose.”

The researchers theorize that the botos’ habitat is what caused them to evolve towards calls with shorter durations, since the echoes of longer calls would be subject to more interference in rivers than they would in oceans. That complex habitat might also explain why the acoustic characteristics of the river dolphin’s calls is somewhere between the low-frequency calls used by baleen whales to communicate over long distances and the high-frequency calls used by marine dolphins for short-distance communication. “There are a lot of obstacles like flooded forests and vegetation in their habitat, so this signal could have evolved to avoid echoes from vegetation and improve the communication range of mothers and their calves,” May Collado said.

The river dolphins made longer calls and whistles, as well, but these sounds were much rarer and the researchers have yet to deduce the dolphins’ uses for them. However, Melo-Santos and co-authors write in the study that botos use these longer whistles in much different social contexts and for what appears to be quite different purposes than their ocean-dwelling cousins: “keeping distance between each other, rather than promoting social interactions as in marine dolphins.”

The researchers say they next plan to study whether or not other populations of Araguaian river dolphins that have had less contact with humans employ the same diversity of calls, and then compare the results with the dolphin’s closest relatives. Two species, the Bolivian river dolphin and the Amazon river dolphin, are closely related to Araguaian dolphins, yet their vocal patterns are substantially different. Amazon dolphins in Ecuador, for instance, which May Collado studied in 2005, are not nearly as talkative.